The prior art already includes monitoring devices which operate on the basis of observing the frequency of a signal derived from the microprocessor circuit, and generating an error signal when the frequency moves outside an allowed range of frequencies extending between a lower threshold and an upper threshold.
Several types of derived signal can be envisaged for detecting operating defects having various causes. Particular mention may be made of the following:
a change in the system clock signal, e.g. due to the oscillator loosing synchronization;
a program running an endless loop;
a special subprogram being run which is dedicated to producing the above-mentioned derived signal; and
the microprocessor operating an unknown instruction.
Depending on the type of derived signal that is used, the prior art monitoring device is capable of generating a signal indicating that the corresponding operating fault has occurred.
However, all prior devices are disadvantageous in that they are constituted by electronic circuits that are complex and therefore expensive.
In addition, these devices have high electricity consumption, and this aspect becomes of vital importance when monitoring a microprocessor system mounted on board a vehicle and powered by the battery thereof. It is essential to maintain the battery at a suitable level of charge even when the monitoring device is in service for a long period of time.
The present invention seeks to mitigate these drawbacks of the prior art and to provide a monitoring device of the type mentioned above which is extremely simple and cheap in structure and which consumes very little electricity.